1. Field of the Invention
This invention relates to the field of compact devices for remotely detecting the presence of chemical or biological agents using an electromagnetic microcavity element or an array or assembly of microcavity elements.
More particularly, it pertains to the devices which detect chemical or biological agents using a state-selective material which is placed inside or surrounding the microcavities. The complex dielectric constant of the microcavities is modified by the presence of the compound to be detected. This invention allows one to detect the presence of chemical and biological agents even at a very low concentration.
In other terms, this invention pertains to a photonic bandgap crystal, the dispersion characteristic of which are modified by the introduction of a chemically or biologically active material, followed by the detection of such modification. The changes in the cavity in the presence of the chemical or biological species can be detected using optical, infrared, or RF probe beams, or a combination thereof.
2. Description of the Related Art
A number of techniques have been tried in prior art for detection of chemical and/or biological agents at low concentrations. For instance, single-pass absorption cell techniques have been used for species classification. Multi-pass cells are also usable for the detection of the species at low concentration.
The simplest example of a multi-pass cell is the White Cell, which consists of a pair of mirrors or diffractive elements that enable a probe beam to reflect multiple times through the same cell volume, enabling one to detect dilute quantities of a substance.
However, the standard White Cell is much larger than the microcavities of this invention, and it can be difficult to tune a large cavity to a precise resonance frequency. In addition, a White Cell typically has a lower number of passes through the sample (on the order of 10 to 100), whereas one of the attractive features of this invention is, as shown below, that a microresonator of this invention can have up to 10,000,000 passes.
Another kind of technique to make a highly selective chemical sensor is taught in U.S. Pat. No. 5,910,286 to Lipskier. Lipskier discloses a chemical sensor having an acoustic wave transducer and a layer of a molecular fingerprint material, the latter comprising a sensitive layer making the sensor highly selective. This material is a macroporous cross-linked product having cavities steric and functional configuration of which is specifically suited to capturing molecular or ionic species, or both. Lipskier teaches how to make the selective material capable of capturing the compound to be detected via an absorption or adsorption process.
Other selective surfaces have also been described. For example, use of polymers as such selective surfaces was described by D. Bucher, et. al. in xe2x80x9cDetection of Influenza Viruses Through Selective Adsorption and Detection of the M-protein,xe2x80x9d J. Immunol. Methods, 96, p. 77 (1987). Use of ceramics was disclosed by R. Diefes, et. al. in xe2x80x9cSample/Reagent Adsorption on Alumina Versus Pyrex Substrates of Microfabricated Electrochemical Sensors,xe2x80x9d Sensors and Actuators, B30, p. 133 (1996). Use of complex organic compounds was taught by J-F. Lee, et. al. in xe2x80x9cShape-Selective Adsorption of Aromatic Molecules from Water by Tetramethylammonium Smectite,xe2x80x9d J. Chem. Soc. Faraday Trans., I85, p. 2953 (1989). Finally, use of membranes was described by D. Petsch, et. al. in xe2x80x9cMembrane Adsorbers for Selective Removal of Bacterial Endotoxin,xe2x80x9d J. Chromatography B693, p. 79 (1997).
However, neither Lioskier nor Bucher, Diefes, Lee or Petsch discuses the electromagnetic cavity resonance effects which are extremely important in detection of even trace amounts of the compound in question.
U.S. Pat. No. 5,907,765 to Lescouzeres, et. al. discloses a method of patterning a cavity over a semiconductor device in order to manufacture a chemical sensor. This method involves forming a sacrificial layer over a substrate followed by patterning and etching this layer so that a portion of it remains on the substrate. The substrate and the remaining portion of the of the sacrificial layer are then covered by an isolation layer over which a conductive layer is formed. The conductive layer serves a purpose of providing a heater for the sensor device. The remaining portion of the of the sacrificial layer is then selectively etched away forming a cavity between the isolation layer and the substrate. This cavity provides thermal isolation between the heater and the substrate.
Lescouzeres, et. al. do teach how to form a cavity, but the purpose of the cavity is thermal isolation. Lescouzeres, et. al. do not use the cavity for enhancement of the probe electric field. Nor do they make any reference to selectivity of frequency or electromagnetic enhancements.
U.S. Pat. No. 5,866,430 to Grow discusses methods and devices for detecting, identifying and monitoring chemical or microbial species using the techniques of Raman scattering. Grow""s methodology includes four steps: (a) the gas or liquid to be analyzed or monitored is brought into a contact with a bioconcentrator, the latter being used for binding with the species or for collection or concentration of the species; (b) the bioconcentrator-species complex is irradiated at one or more predetermined wavelengths to produce the Raman scattering spectral bands; (c) the Raman spectral bands are processed to obtain an electric signal; and (d) the electric signal is processed to detect and identify the species, quantitatively, qualitatively, or both.
The Grow invention uses a Raman Optrode instrument comprising a Raman spectrometer capable of collecting and processing the Raman scattering spectral information and converting it into electrical signals. This method uses Raman Spectroscopy for the analysis. It teaches the use of a bioconcentrator which utilizes adsorption and absorption techniques. However, Grow does not disclose any use of the field enhancement cavities.
U.S. Pat. No. 5,835,231 to Pipino discloses a broadband, ultra-highly sensitive chemical sensor which detects chemicals through the use of a small, extremely low-loss, monolithic optical cavity fabricated from highly transparent, polygonally shaped optical material. Optical radiation in this invention enters and exits the monolithic cavity by photon tunneling in which two totally reflecting surfaces are brought in a close proximity. In the presence of an absorbing material, the loss per pass is increased and the decay rate of an injected pulse is determined. The change in decay rate is used to obtain a quantitative sensor with sensitivity of 1 part per million per pass or better. A similar idea was also described by A. Pipino in xe2x80x9cUltrasensitive Surface Spectroscopy with a Miniature Optical Resonator,xe2x80x9d Phys. Rev. Let., Vol. 83, No. 15, p. 3093 (1999).
Pipino does use the concept of optical field enhancement in a cavity; however, he uses only a single microcavity and an array. Thus, Pipino does not allow the enhancement effect to occur over a broad area, nor does he teach any means of attracting or concentrating the species to be detected.
Finally, U.S. Pat. No. 5,744,902 to Vig discloses a chemical or biological sensor formed from a coated array. Both mass and temperature changes due to the presence of a particular substance or agent causes a change in output frequency, which change is linked to the analyzed species. Furthermore, the change in frequency output due to the mass loading is distinguished from the change due to the temperature change. Vig teaches arrays of microresonators; however, his resonators are mechanical and not electromagnetic ones.
However, the subsequently discussed microresonators of this invention, have serious advantages compared to those of the Vig""s invention. Probing the microcavities optically is easier, the sensitivity may be greater, and this invention offers a means to probe remotely, using an optical or RF-beam. Vig does not have such remote probing feature.
There is a need to have compact, low cost remote sensors of chemical and/or biological species which:
(a) are very sensitive in proportion to their compact size and are able to detect very small quantifies of the compound in question;
(b) can be scaled to function in the visible portion of the spectrum, throughout the infra-red portion and into the teraherz or microwave region;
(c) can be easily fabricated using standard photolithographic techniques on a variety of substrates;
(d) can be fabricated as a monolithic planar devices integrated into a waveguide structure, or configured as volumetric sensors;
(e) are lightweight;
(f) can be employed on a unmanned air vehicle (UAV) or xcexc-UAV platforms for remote sensing;
(g) can detect multiple resonances within a substance, or multiple substances;
(h) can be made to ignore false positive results (anti-spoofing);
(i) can be made to have self-calibrating capabilities;
(j) can have a larger lifetime and a higher production yield; and
(k) are expendable.
Compact sensors using microcavity structures satisfy all these requirements. Previously, known sensors required long interaction lengths to enable detection of small amounts of a given species. Therefore, there was a need for cumbrous white-cell configurations where the substance to be detected is to pass the structure multiple times.
The concept of the state-selective microcavity array leads to production of a novel biochemical sensor. As will be shown below, the present invention avoids problems associated with previously known sensors by using electromagnetic cavity resonance effects and by enhancing the electromagnetic field of the species being analyzed.
There exists no known prior art for compact sensors using microcavities for enhancement of the probe electromagnetic field. Yet the need for such is acute.
For the foregoing reasons, there is a necessity for a compact low-cost sensor for detection of very low amounts of chemical and/or biological substances using microcavities. The present invention discloses such sensors.
The present invention is directed to a compact sensor of chemical and/or biological agents using microcavities. The agent to be detected passes the microcavities and modifies the properties of the microcavities, or, in another embodiment, is capable of being detected because the sensitivity of the device is greatly enhanced by the microcavity.
When attempting to detect a biological or chemical species, one crucial factor is the sensitivity of the detector to dilute concentrations, because some of most dangerous biological toxins can be lethal at levels of only a few parts per billion, and bacteria or viruses can achieve infection with a very small number of organisms. These minuscule doses needed to achieve a lethal outcome lead to a challenging problem in detection, as the human detector may reveal symptoms at concentrations far below what an electronic detector or other classes of sensors can register.
In optical, infrared, or millimeter wave sensing, detection often means sensing a change in the amplitude or phase of a wave which is passing through or reflecting off of the material under test. Examples include passing a probe beam through the air, or reflecting the wave off of the ground or other surfaces, and looking for particular absorption lines.
If the substance to be detected is very dilute, its effects may fall below the noise floor of the detection system. One can address this problem by sampling the material many times with a single probe beam, such as in an optical cavity. Any absorption or phase shift of the probe beam will be effectively multiplied by the Q factor of the cavity, resulting in a stronger signal.
To achieve cavity enhancement of a visible, infra-red, or millimeter wave signal does not necessitate having large cavity structures with precisely aligned mirrors. The same effects can be seen in many naturally occurring forms. For example, in surface-enhanced Raman spectroscopy, as little as a few molecules can be detected simply by adsorbing them to a metal surface. The natural roughness of the metal surface creates micron-scale hills and valleys which can be seen as tiny optical cavities.
When a molecule falls into one of these natural microcavities, the electric field of the probe beam is enhanced by the walls of the cavity resulting in a much stronger received signal. This is equivalent to sampling the same molecule many times with a single probe beam. Although Raman spectroscopy relies on non-linear optical effects the same enhancement also applies to linear effects such as optical absorption.
While a rough metal surface clearly provides some absorption enhancement to chemicals on the surface, it does so in a random, uncontrolled manner. If one wishes to sense a variety of biological or chemical species, each with a distinct electromagnetic signature, a broadband source is required. With a random assortment of natural microcavities, the received signal would be an unintelligible spectrum containing a superposition of the electromagnetic signatures of all nearby compounds or organisms.
A more sensible approach involves applying this knowledge of microcavity electromagnetic enhancement with modern fabrication techniques to create a detector with well-defined properties. Such a detector would consist of an array of microcavities which would be designed to sense only a single absorption line of a particular species or chemical, or a set of well-defined spectral features which, collectively, act as a unique xe2x80x9cfingerprintxe2x80x9d of the species to be detected.
This would be achieved by selecting the resonance frequency of the microcavities to coincide with a resonance of the material to be detected. The selectivity of the detector is enhanced if it is coated with a gel which selectively adsorbs certain chemicals or organisms, while rejecting others. By combining many such arrays with different resonance frequencies into a single detector, it could detect a variety of different species. Integrated with electronic logic circuits, this detector would be insensitive to xe2x80x9cfalse positivexe2x80x9d readings from other substances. The entire sensor array could be produced using photolithographic and MEMS processing techniques, and assembled into a chip-scale package, with many of the components residing on a single monolithic substrate.
A microstructure possesses a Q characteristic which can be defined in a number of ways. Q is a ratio between energy stored inside cavities and energy lost per cycle. For the purposes of this invention Q can also be interpreted as
Q=Ec2/Eb2, 
where Ec is the electromagnetic field inside the cavities and Ebis electromagnetic field of the probe beam.
Q can also be looked at as a number of equivalent passes of the probe beam inside the cavity, for instance, the number of times the sample is probed.
The effective cavity Q of the microstructure element is large. Given this fact, the sensitivity of the structure is enhanced compared to conventional approaches.
The smallest size of the microcavities is on the order of one cubic half-wavelength. Generally, the degree of porousness for the microcavities of this invention is about one microcavity per square wavelength on a two-dimensional structure or one microcavity per cubic wavelength on a tri-dimensional structure. The size of the pores is generally smaller than a cubic half-wavelength and is related to the Q of the microcavities. If r is a radius of the microcavities and xcex is a wavelength, then Qxcx9c(xcex/r)3.
In one aspect, the present invention provides a process of building electromagnetic structures having microcavities. These structures have well-defined operating frequencies which are adjustable by varying the physical parameters of the cavities according to a known set of design parameters. Combining these microcavities with the state-selective absorbents, a sensor is fabricated, which first attracts and concentrates the bio-chemical substance to be measured, and then detects it with a high decree of sensitivity through the cavity enhancement effect.
The entire system is amenable to chip-scale integration with microelectronic circuits to create an intelligent sensor in a small package.
Microcavities are preferably manufactured in a form of an array. Such an array is useful because it provides a large area of microcavities. In principle, a single microcavity, especially a treated microsphere subsequently discussed, can also be used. However, an array of microcavities is preferable, because a single cavity could be more difficult to probe because of its small size.
A microcavity array may be fabricated on a surface of a passive planar structure which also contains a state-selective adsorbing material. The state-selective adsorbing material is any material which changes in response to the presence of the material to be detected. For instance, a state-selective material can be an antibody of a particular antigen which one is trying to detect.
A chemical or biological species is adsorbed or absorbed by the treated material in the structure. As a result, a transmission and/or reflectivity of the structure, determined by a probe beam, is modified in phase and/or amplitude due to the presence of a given chemical or biological species.
The basic structure comprises an array of micro-resonators. This structure can also be classified as a photonic bandgap crystal with a state-selective absorbing material. The structure may be in the form of a two-dimensional planar array of these elements or may be a three-dimensional volumetric ensemble of such microcavities. The device can be fabricated as a monolithic structure or individual micro-spheres or disks can be self-assembled onto a common substrate or attached to the end of an optical fiber bundle or RF waveguide. Yet another device architecture is a pair of waveguide channels, between which is situated a microsphere or a micro-ring resonator.
The microcavity array can also be in a form of an ensemble of micro-resonators, micro-disks, or micro-spheres which can be fabricated onto a common substrate using self-assembly techniques. Other three-dimensional structures can be likewise used.
Such self-assembly techniques are well known to those skilled in the art. They comprise, for example, the technique for assembly of mono-layers of dielectric spheres, in which a substrate is drawn out of a liquid, usually water, containing the spheres. The spheres then form a mono-layer on the substrate.
The system can be used for spectral analysis in the visible range, in the infra-red range or in the teraherz range, depending on the scale size of the microcavity elements. Multiple-sized elements can be easily integrated onto the same substrate for hyper-spectral analysis of more complex compounds. In such case, a more detailed spectroscopic evaluation may be needed to distinguish between similar species, which is very important since certain classes of toxic species have spectral and structural properties similar to those of their non-toxic analogs.
The array can be probed using various differential detection methods for common-mode rejection of source and environmental noise, resulting in a more robust sensor package. Various techniques can be used for improved performance, including, but not limited to, differential absorption, modulation spectroscopy, or frequency-shifted sources. These methods and techniques are well known to those skilled in the art of spectroscopy.
This patent discloses two related systems for detection of species, both of which systems are described below in detail. According to the first system, the species modifies the microcavity Q, and such modification is detected by absorption of a resonant or near-resonant probe beam. According to the second system, the microcavity is not modified, but the species affects the phase shift which is detected by sampling of the structure by a resonant or non-resonant probe, the sensitivity of which is enhanced by the high Q of the structure.
For the purposes of this disclosure, the term xe2x80x9cresonancexe2x80x9d is defined as a condition where the wavelength of the probe beam matches the absorption lines of the state-selective material.
Each of the two systems mentioned above can be implemented by more than one embodiment.
One aspect of this invention provides a sensor for detecting chemical and/or biological compounds comprising a first element comprising a plurality of microcavities disposed on a substrate, and a second element comprising a source of electromagnetic radiation and a detector of electromagnetic radiation, wherein the chemical and/or biological compounds are adsorbed or/and absorbed by the microcavities causing a change of electromagnetic field of the microcavities, and the change being detected by the second element.
Another aspect of this invention provides a method for detecting chemical and/or biological compounds, comprising steps of providing a substrate with a plurality of microcavities disposed thereupon, providing a probing device comprising a source of electromagnetic radiation and a detector of electromagnetic radiation, directing the chemical and/or biological compounds at the microcavities, adsorbing or/and absorbing the chemical and/or biological compounds by the microcavities causing a change of electromagnetic field of the microcavities, and detecting the change of electromagnetic field by the probing device.